Experiments in choloralose-anesthetized dogs indicated that the degree of respiratory variations in pulse interval (VHP) correlated linearly with the degree of parasympathetic cardiac heart rate control (PC), when the latter was defined as the change in pulse interval caused by the elimination parasympathetic efferents while keeping sympathetic efferent activity to the heart unaltered. The overall goal of the project is to determine under what physiological conditions this relationship holds. If it is shown that the relationship holds under a wide range of conditions, then the noninvasively measurable VHP could be used as an indicator of PC, otherwise obtainable invasively only. The objectives for the current year of this project are to concentrate on chronically prepared animals and to determine the relationship between VHP and PC not only during rest (as primarily done during the first year) but also during exercise. The effect of sympathetic blockade on the relationship between VHP and PC is also to be investigated, and the measured slope between the two variables is to be compared with the slope predicted by theoretical considerations that involve respiratory timing.